BUG/MEDIUM: Make sure stksess is properly aligned.

BUG/MEDIUM: Make sure stksess is properly aligned.

When we allocate struct stksess, we also allocate memory to store the
associated data before the struct itself.
As the data can be of different types, they can have different size. However,
we need the struct stksess to be properly aligned, as it can do 64bits
load/store (including atomic load/stores) on 64bits platforms, and some of
them doesn't support unaligned access.
So, when allocating the struct stksess, round the size up to the next
multiple of sizeof(void *), and make sure the struct stksess itself is
properly aligned.
Many thanks to Paul Martin for investigating and reporting that bug.

This should be backported to earlier releases.